In recent years there has been much interest in identifying nucleoside inhibitors of the NS5B polymerase for treatment of hepatitis C virus (HCV) infection. While initial reports from researchers in this area described modest efficacy, the tremendous potential of this class of agents became clear when Merck presented the results of evaluation of MK-0608 in the chimpanzee. (Olsen, D. B.; Carroll, S. S.; Davies, M. E.; Handt, L.; Koeplinger, K.; Zhang, R.; Ludmerer, S.; MacCoss, M.; Hazuda, D. J. Robust suppression of viral replication in HCV infected chimpanzees by a nucleoside inhibitor of the NS5B polymerase. Antivir. Ther. 2006, 11 (5, Suppl.): S7. (15th International HIV Drug Resistance Workshop, Jun. 13-17, 2006; Sitges, Spain.)) In this study, a >5 log10 reduction in viral titer was achieved in 7 days at an intravenous dose of 2 mg/kg/day.
Other agents advanced into development have not achieved such dramatic efficacy. For example, NM283 (Idenix, recently discontinued) achieved only a 1.15 log10 reduction in viral titer in the chimpanzee (7 days, 16.6 mg/kg/day). (Standring D. N.; Lanford R.; Wright T.; Chung R. T.; Bichko V.; Cretton-Scott E.; Pan-Zhou X.; Bergelson S.; Qu L.; Tausek M.; Bridges E.; Moussa A.; Storer R.; Pierra C.; Benzaria S.; Gosselin G.; La Colla P.; Sommadossi J. P. NM 283 has potent antiviral activity against genotype 1 chronic hepatitis C virus (HCV-1) infection in the chimpanzee. J. Hepatology, 2003, 38, (Supp 2), 3.) In a phase 2 study, R1626 (Roche) at 1500 mg BID for 14 days achieved a mean reduction in serum viral titer of 1.2 log10. (Roberts, S.; Cooksley, G.; Shaw, D.; Berns, H. K.; Brandl, M. T.; Fettner, S. H.; Hill, G.; Ipe, D.; Klumpp, K.; Mannino, M.; O'Mara, E.; Tu, Y.; Washington, C. B., Interim results of a multiple ascending dose study of R1626, a novel nucleoside analog targeting HCV polymerase in chronic HCV patients. 41st Annual Meeting of the European Association for the Study of the Liver, Vienna, Apr. 26-30, 2006.)

The difference in efficacy between these agents, all of which display good plasma levels of the nucleoside, is likely due to differences in rates of phosphorylation and resulting levels of triphosphate in the liver. One method of circumventing a slow rate of initial nucleoside phorphorylation is to utilize a prodrug of the nucleoside monophosphate (kinase bypass). A number of prodrugs have been explored for this purpose, although few have been shown to achieve oral bioavailability and intracellular delivery of the monophosphate in vivo.
One such class of prodrugs is the aryl amidate (McGuigan) type. While these prodrugs have shown impressive intracellular delivery of monophosphates in vitro, there are few reports of in vivo application. (By contrast, their track record with phosphonates has been better.) One notable report by McGuigan details pharmacokinetic evaluation in the cynomolgus monkey of an aryl amidate prodrug of abacavir. (McGuigan, C.; Harris, S. A.; Daluge, S. M.; Gudmundsson, K. S.; McLean, E. W.; Burnette, T. C.; Marr, H.; Hazen, R.; Condreay, L. D.; Johnson, L.; De Clercq, E.; Balzarini, J. Application of phosphoramidate pronucleotide technology to abacavir leads to a significant enhancement of antiviral potency. J. Med. Chem. 2005, 48, 3504-3515) This article reports extremely rapid clearance of the prodrug from plasma when administered i.v. The dearth of other literature reports of in vivo characterization of aryl amidate prodrugs, despite numerous applications citing in vitro characterization, suggests that these prodrugs are not sufficiently stable for successful in vivo application.
An alternative class of monophosphate prodrugs that target the liver are the cyclic 1-aryl-1,3-propanyl ester (HepDirect) prodrugs. (Erion, M. D.; Reddy, K. R.; Boyer, S. H.; Matelich, M. C.; Gomez-Galeno, J.; Lemus, R. H.; Ugarkar, B. G.; Colby, T. J.; Schanzer, J.; Van Poelje, P. D. Design, synthesis, and characterization of a series of cytochrome P(450) 3A-activated prodrugs (HepDirect prodrugs) useful for targeting phosph(on)ate-based drugs to the liver. J. Am. Chem. Soc. 2004, 126, 5154-5163; Erion, M. D.; van Poelje, P. D.; Mackenna, D. A.; Colby, T. J.; Montag, A. C.; Fujitaki, J. M.; Linemeyer, D. L.; Bullough, D. A. Liver-targeted drug delivery using HepDirect prodrugs. J. Pharmacol. Exp. Ther. 2005, 312, 554-560) One HepDirect phosphate prodrug, MB07133, has been advanced to human clinical trials. (Boyer, S. H.; Sun, Z.; Jiang, H.; Esterbrook, J.; Gomez-Galeno, J. E.; Craigo, W.; Reddy, K. R.; Ugarkar, B. G.; MacKenna, D. A.; Erion, M. D. Synthesis and characterization of a novel liver-targeted prodrug of cytosine-1-β-D-arabinofuranoside monophosphate for the treatment of hepatocellular carcinoma. J. Med. Chem. 2006, 49, 7711-7720). MB07133 is a prodrug of cytarabine 5′-O-monophosphate (araCMP).

Scientists at Roche, in collaboration with McGuigan at Cardiff University, recently reported the synthesis and evaluation of aryl amidate monophosphate prodrugs of 4′-azidouridine. (Perrone, P.; Luoni, G. M.; Kelleher, M. R.; Daverio, F.; Angell, A.; Mulready, S.; Congiatu, C.; Rajyaguru, S.; Martin, J. A.; Leveque, V.; Le Pogam, S.; Najera, I.; Klumpp, K.; Smith, D. B.; McGuigan, C. Application of the phosphoramidate ProTide approach to 4′-azidouridine confers sub-micromolar potency versus hepatitis C virus on an inactive nucleoside. J. Med. Chem. 2007, 50, 1840-1849) Whereas R1479 (the nucleoside parent of R1626) achieves reasonable potency in the cell-based replicon assay, 4′-azidouridine is inactive despite similar potency of its triphosphate as an inhibitor of RdRp. (Smith, D. B.; Martin, J. A.; Klumpp, K.; Baker, S. J.; Blomgren, P. A.; Devos, R.; Granycome, C.; Hang, J.; Hobbs, C. J.; Jiang, W.-R.; Laxton, C.; Le Pogam, S.; Leveque, V.; Ma, H.; Maile, G.; Merrett, J. H.; Pichota, A.; Sarma, K.; Smith, M.; Swallow, S.; Symons, J.; Vesey, D.; Najera, I.; Cammack, N. Design, synthesis and antiviral properties of 4′-substituted ribonucleosides as inhibityors of hepatitis C virus replication: the discovery of R1479. Bioorg. Med. Chem. Lett. 2007 17:2570.) Certain aryl amidate monophosphate prodrugs of 4′-azidouridine display replicon activity (EC50 as low as 0.22 μM), indicating that the first phosphorylation step is the problematic one. In vivo evaluation of these prodrugs was not reported.
R1479 (4′-AC) 4′-azidouridine (4′-AU) R0142 NS5b IC50 of NTP (μM)Replicon EC50 (μM)R14790.291.284′-AU0.30>100R0142—0.22